Polyvinyl alcohol type polymers, particularly ethylene-vinyl alcohol copolymers (hereinafter, merely referred to also as “EVOH”) exhibit extremely small amount of oxygen transmission in comparison with other plastics, and favorable melt formability; therefore, they have been extensively used as food packaging materials and other packaging materials. However, when packaging materials produced using EVOH or the like are subjected to a retorting process under high-temperature and high-humid conditions, blooming and deformation may occur, or barrier properties may be deteriorated, and thus improvement of retort resistance has been demanded.
As a strategy for improving retort resistance and the like of packaging materials in which EVOH is used, various techniques in which EVOH is crosslinked using an activated energy ray such as an electron beam have been proposed. For example, Patent Document 1 discloses a method including melting and kneading triallyl cyanurate or triallyl isocyanurate used as a crosslinking agent, with EVOH, followed by irradiating with an electron beam to permit crosslinking of EVOH.
In addition, Patent Document 2 and Patent Document 3 disclose a procedure in which at least one crosslinking agent and/or crosslinking activator selected from a polyfunctional allylic compound, a polyfunctional (meth)acrylic compound, a polyhydric alcohol and a metal oxide is added to EVOH, and the mixture is irradiated with an electron beam to permit crosslinking.
Furthermore, Patent Document 4 describes that a compound having two or more allyl ether groups is added to EVOH, followed by irradiation with an electron beam to permit crosslinking.
Moreover, Patent Document 5 discloses a method in which an unmodified ethylene-vinyl alcohol type copolymer (A) is modified with an epoxy compound (B) having a double bond and an epoxy compound (E) not having a double bond, and at least a part of the obtained modified ethylene-vinyl alcohol type copolymer (C) is crosslinked with an electron beam.
However, according to the strategies of Patent Document 1 to Patent Document 4, the crosslinking agent remained in the resulting crosslinked product may bleed out. When such bleeding out occurs, hygienic problems are concerned particularly when a crosslinked product is used for food packing containers. In addition, when a multilayer film is produced by laminating a film obtained using the resulting crosslinked product, interlayer adhesiveness may be insufficient due to the bleeding out occurred, and yet the retort resistance can be further improved. Whereas, according to Patent Document 5, an especial extruder is needed for modifying EVOH, thereby leading to a problem of poor versatility.